Stem/progenitor cell proliferation and differentiation must be tightly regulated to maintain appropriate cell numbers for normal organ function while preventing tumorigenesis. Elucidation of mechanisms that regulate somatic stem/progenitor cell homeostasis is key to understanding how these populations are maintained and activated to meet demands for tissue regeneration following injury. Intercellular junctions, comprised of tight junctions (TJ), adherens junctions (AJ) and desmosomes, are sites of intercellular adhesion. Claudins are integral TJ proteins that regulate paracellular permeability. Claudin 18 (C18) is one of the most highly expressed Claudin family members in lung alveolar epithelium. We recently generated C18 knockout (KO) mice that exhibit increased lung epithelial permeability to ions and solutes. Intriguingly, C18 KO mice show expansion and increased proliferation of putative lung stem/progenitor cells (including alveolar epithelial type II (AT2) cells) and increased lung (and stomach) size, implicating integral TJ proteins (and C18 in particular) as novel regulators of epithelial stem/progenitor cell homeostasis and organ size. The Hippo signaling pathway regulates stem/progenitor cell function, organ size and regeneration through opposing effects on proliferation and apoptosis. Hippo signaling mediates contact inhibition of cell proliferation in vitro and limits tissue overgrowth in vivo via phosphorylation of upstream kinases that inhibit activity of orthologous downstream transcriptional co-activators, yes-associated protein (YAP) and transcriptional coactivator with PDZ binding motif (TAZ). Upstream regulators of Hippo signaling are not well characterized but include extracellular stimuli via G-protein coupled receptors, actin cytoskeleton, apical-basolateral polarity complexes and interactions with TJ-associated and AJ proteins. Integral TJ proteins (e.g., claudins) have not previously been shown to regulate Hippo signaling or YAP/TAZ activity. Preliminary studies demonstrate YAP/TAZ activation in C18 KO mice and association of YAP with C18 in wild type mice, leading us to hypothesize that C18 is a novel regulator of lung stem/progenitor cell homeostasis via modulation of YAP/TAZ subcellular localization/activity. The overall goal of this project is to investigate the role of C18 in regulating lung stem/progenitor cell homeostasis by addressing the following Specific Aims: 1) explore cellular mechanisms underlying lung phenotype of C18 KO mice: 2) investigate signaling mechanisms regulating stem/progenitor cell homeostasis in C18 KO mice; and, 3) characterize molecular mechanisms whereby C18 regulates YAP/TAZ signaling. Elucidation of mechanisms regulating lung stem/progenitor cell function and identification of novel pathways transducing growth-promoting signals from TJ to the nucleus have important implications for modulating stem/progenitor cell function and augmenting regeneration following lung injury.